This invention relates to apparatus for detecting degradation of the characteristics of an arrester, more particularly of the type utilizing a stack of zinc oxide type nonlinear resistors as a characteristic element.
The characteristic elements or the nonlinear resistors utilized in the prior art arrester have generally been of the SiC type. However, due to insufficient nonlinearity, the SiC type characteristic elements could not be used for arresters not provided with series gaps.
In recent years metal oxide type resistors, more particularly zinc oxide type resistors, having a large nonlinear coefficient .alpha. (which is expressed by an equation I=KV.sup..alpha., where V represents voltage, K a constant and I current) have been developed for eliminating the series gap. In an arrester utilizing a characteristic element made up of a stack of such metal oxide type disc shaped resistors, its characteristics should not vary appreciably because degradation thereof results in the increase in the normal leakage current and temperature rise causing thermal rupture.
For this reason, it is necessary to seasonally check the characteristics of the zinc oxide type resistor elements.
The characteristic check of an arrester is usually performed by a circuit shown in FIG. 1. As shown, an arrester constituted by a series gap G and a nonlinear resistor R.sub.1 is grounded through a resistor r.sub.1 and the voltage drop across the resistor r.sub.1 is measured to check the variation in the characteristics of the nonlinear resistor R.sub.1. As shown in an equivalent circuit shown in FIG. 2, in an arrester utilizing zinc oxide nonlinear resistors R.sub.2, each nonlinear resistor R.sub.2 of the characteristic element is shunted by an electrostatic capacitance C.sub.2 so that the method of measuring the voltage drop across a resistor r.sub.2 connected between the arrester and the ground is not effective.
Thus, the phase relationship among the impressed voltage e and the currents i.sub.C2 and leakage current i.sub.R2 which flow through the electrostatic capacitance C.sub.2 and the nonlinear resistor R.sub.2 respectively is shown in FIG. 3. The factor that produces Joule heat which accelerates degradation of the nonlinear resistor R.sub.2 is attributable to the leakage current i.sub.R2 flowing therethrough. However, as shown in FIG. 3 the capacitive leakage current i.sub.C2 and the resistive leakage current i.sub.R2 have a phase difference of 90.degree. and their crest values have a relationship of i.sub.C2 &gt;i.sub.R2.
Accordingly, even when the leakage current of the arrester is detected by using resistor r.sub.2 the resistive leakage current i.sub.R2 which is essential to determine the degree of degradation of the arrester characteristics would be masked by the capacitive leakage current i.sub.C2. Thus, it is difficult to measure the exact value of the resistive leakage current.
Various methods have been proposed to eliminate the capacitive leakage current i.sub.C2. According to one method, a condenser is connected to the line terminal of the zinc oxide type arrester to pass current having the same phase as the capacitive leakage current i.sub.C2 of the zinc oxide arrester. The phase of the leading current flowing through the condenser is shifted 180.degree. by a phase shifter to match its phase with that of the capacitive leakage current i.sub.C2. Then the two currents are added together after adjusting the crest value of the leading current to be equal to that of the capacitive leakage current i.sub.C2. This method assures accurate measurement of the resistive leakage current i.sub.R2.
This method, however, is not advantageous in that it is necessary to use an additional capacitor for the purpose of obtaining a waveform that cancels the charging current of the arrester, that the grounding connection must be interrupted temporarily, and that the measuring procedure is troublesome.